In an air-conditioning system, generally, there are two methods of adjusting the room temperature due to fluctuations of the heat load inside the room to maintain it a fixed level. Either regulate only the air temperature of the air supplied while the air flow rate is kept at a fixed level or vary only the amount of air blown out. To accomplish this purpose, it is essential in the former case that the amount of air blown out be accurately kept at a fixed level and in the latter case, it is essential that the air flow rate which varies from time to time maintained at a fixed value irrespective of pressure fluctuations inside the duct at each such point of time. In practice, however, the flow rate of conditioned air from the central air conditioner, which blows out into the room through the duct, is afffected by fluctuations in the wind pressure inside the duct. In other words, with the increase and decrease in the wind pressure inside the duct, the wind pressure is converted into velocity head near the outlet. Thus, the air flow rate increases or decreases, and it is not always possible to maintain it at a prefixed level. To maintain a constant rate of air flow irrespective of such fluctuations of the wind pressure inside the duct is the problem to be solved.
In the conventional type of automatic control device for maintaining the air flow rate at a certain fixed level the mechanism is such that the wind pressure inside the duct, the total pressure, is received by a pressure-receiver plate, and by the balancing the reaction of the pressure-reeceiver plate and the stress of a mechanical spring, an air-throttling locomotor is shifted, thereby opening and closing the air passage by degrees to increase and decrease its area. The total pressure inside the duct is the sum of static and dynamic pressures, and of the two pressures, the dynamic pressure is liable, to deflect in the lengthwise or crosssectional direction of the duct, and change its value by a large margin depending the shape of the duct or upon the wind velocity inside the duct, thus displaying a strong tendency for channeling and pulsing. Consequently, the total pressure has the same tendency as the said dynamic pressure for channeling and pulsing, thereby making the action of the aforementioned air-throttling locomotor unstable. This constitutes a shortcoming of the conventional type of automatic air flow rate controller.